Pigeons are survivors, man. It might be that the virus has mutated and won't spread further, and only their descendants are sentient/nearly. It won't affect any other species of pigeon or other birds, and even if it does spread it won't change all pigeons.

Here is a bit of HB's lore in a bite-sized piece! One thing that's left out - these go from normal pigeons to writing a declaration of independence in five years.